In a first known ventilation system, the cooling gas flows in axially at each of the two ends of the rotor through lower cavities disposed under main cavities in which the winding is installed. (Here, the expression "below the cavities" means "on the rotor axis side of the cavities" and likewise, the expression "upper part" and "lower part" or the like designate hereinafter the parts situated nearest and furthest from the axis respectively). These lower cavities distribute cold gas in the regularly spaced radial passages formed directly in the copper conductors of the winding.
As it flows radially along the cavity, the gas removes heat lost from the induction winding and is hot when it leaves the air gap.
The temperature to which gas is heated as it flows radially and the local difference between the temperature of copper conductors and that of the gas depend on both the amount of heat to be removed and the internal arrangement of the cooling ducts through which the gas flows.
French patent No. 1 449 036 describes a particularly favourable disposition of the radial arrangement of the internal ducts. In said disposition, the cooling ducts are arranged alternately.
French patent No. 2 241 905 (Ganz Villamossagi Muvek) describes a second ventilation system in which the lower parts of the cavity are cooled by the gas which flows in the transverse ducts which are fed from a lower cavity via a central radial duct; the gas flows into the air gap via radial side ducts (the radial direction being that of a straight line which meets the axis of the rotor at right-angles, the longitudinal or axial direction being that which is parallel to the axis of the rotor and the transverse direction being that which is perpendicular to both the longitudinal and radial directions). In the second system, the upper parts of the cavity are cooled by the gas which flows in transverse ducts which are fed in parallel from the air gap via a radial side duct, the gas returning to the air gap via another side duct. The radial side ducts of the lower and upper parts of the cavity form a continuous passage so that in some transverse ducts, the gas which comes from the air gap mixes with that which comes from the lower cavity.
The first ventilation system has a disadvantage related to the fact that the gas which flows in the upper part of the cavity is already heated in the lower part. The disadvantage is that the upper part is less efficiently cooled by an already heated gas than the lower part by a gas which is still cold. Hence, there is a great difference in temperature between the conductors of the upper part and those of the lower part.
The second ventilation system has two important disadvantages. The first is that it is complicated and therefore expensive, since it requires firstly a lower cavity and a system for feeding it and secondly it requires aerodynamic systems to be formed, said aerodynamic systems using the rotation of the rotor to draw off the cooling gas from the air gap and to return it thereto. The second disadvantage is that the proper distribution of the pressure and of the flow of the gas in the ducts depends on the proportion between two magnitudes. The rotation speed of the rotor is the condition for the first of these magnitudes, namely the differential pressure generated by the aerodynamic systems for drawing off and returning the gas of the air gap. The second of the magnitudes is the difference in pressure between the lower cavity and the air gap. In particular, it depends on the distance to be covered by the gas in the lower cavity, from the point where it enters the rotor. In some cases a great relative variation of these two magnitudes can cause a poor distribution of the gas flow and improper cooling of some transverse ducts.
Preferred embodiments of the present invention provide a simple means for increasing the efficiency of the ventilation system while reducing both the average temperature rise of the winding and the temperature difference between the copper conductor at the top of the cavity and the copper conductor at the bottom of the cavity.
The present invention advantageously modifies the disposition of the cooling ducts in the upper part of the cavity while in the lower part of the cavity, the disposition is still that described in French patent No. 1 449 036 or a similar disposition.